The present invention is particularly applicable for inductively heating the three-lobed internal bore or passage in a thin-walled steel workpiece used in the drive assembly of a front wheel driven vehicle and it will be described with particular reference thereto; however, the invention has broader applications and may be used for surface hardening the bore extending through thin-walled workpieces, irrespective of the profile or shape of the bore defining the inner surface. Although the method hardens only selected axially extending areas of the bore, it could be used to harden the total inner surface of the bore.
It is somewhat common practice to harden the inner cylindrical surface of various bores, such as the hub on a wheel, by providing a circular inductor matching the cylindrical inner surface and generally smaller in diameter so that a coupling gap is established between the inductor and the inner surface. This inductor is energized and moved along the cylindrical surface to heat progressively and by induction the cylindrical surface. The surface is hardened by providing a quench body below the inductor which directs a quenching liquid outwardly to quench harden the previously heated portion of the cylindrical surface. This is standard practice and has been used for many years. To provide uniform, progressive heating and quench hardening, it is also known to rotate the workpiece about the central axis of the cylindrical bore so that the progressing inductor and its quench body act upon a rotating cylindrical surface. When this procedure is employed for workpieces having a substantial amount of metal mass around the cylindrical surface, the cylindrical surface is not substantially distorted by the progressive heating and quench hardening. As the mass surrounding the cylindrical surface is decreased to a thin-walled structure, such as where the depth of hardening is about 50% of the total wall thickness, axial distortion often occurs during the progressive hardening procedure. Thus, the cylindrical surface must be ground after hardening if precise dimensions are to be retained. In some instances, even with heavier wall thicknesses, grinding is necessary to obtain a straight cylindrical bore after hardening of the inner surface.
The combined progressive heating and quench hardening together with a subsequent grinding operation has been successful when the thin-walled workpiece has a bore with a cylindrical surface. Extreme difficulties have been experienced when the bore extending through the thin-walled workpiece is not cylindrical. The inner surface of a non-cylindrical bore extending through a thin-walled workpiece cannot be economically ground to correct any distortions in an axial direction. This inability to correct distortion created by progressive hardening of a non-cylindrical bore in a thin-walled workpiece is even more pronounced as the axial length of a bore increases. For these reasons, progressive heating and subsequent quench hardening of an axially extending bore through a thin-walled workpiece, when the bore is non-cylindrical, has not been able to be used when tolerances such as 0.001 to 0.003 inches are to be maintained over a length of four to six inches. Consequently, substantial difficulty has been experienced in surface hardening the internal, three-lobed bore for the outer housing of a front wheel drive mechanism in a motor vehicle. This elongated housing, known generally as a "tripot housing" requires a close tolerance in an axial direction after hardening. Internal machining is not practical.